Create Trigger To Format Text

We are using an old version of Numara TrackIT for our helpdesk software, and it doesn't have much in the way of configurable options. There is no way to set validation or formatting on the text fields in the program.

There is a field, WO_TEXT1, Which I would like to be formatted as 6 characters, 3 integers + a period + 2 integers. The first the integers would be padded with zeros on the left, and the last 2 integers would be padded with zeros on the right.

IE, if someone enters 2, it would actually end up being 002.00
If someone enters 3.5 it would end up being 003.50
If someone enters 12.1 it would end up being 012.10
If someone enters 172.80 it would end up being 172.80

I was hoping to achieve this via an update trigger.

Below is the guts of the trigger I created, mostly as a proof of concept.

-- This update properly formats the Estimated Hours field
Update t SET WO_TEXT1 = (SELECT RIGHT('000000' + CONVERT(VARCHAR(6), WO_TEXT1), 6) FROM inserted)
FROM dbo.TASKS as t
Where (EXISTS (SELECT * FROM inserted WHERE WOID = 24773));

I expected that this update trigger would only affect the Work Order with a WOID of 24773. Unfortunately, it updated all 21000 work orders in our system, wiping out all of the actual estimated hours that had been inserted by technicians!

Luckily I had a report that I could quick dump the 300 or so active work order's estimated hours back into the DB from (all the other Work orders are closed, and no one really cares about their estimated hours).

My question is three fold,

1) Why did my trigger update every record in the tasks table instead of just WO 24773?
2) Is using a trigger the best way of accomplishing what I'm trying to do?
3) if a trigger is the best way of accomplishing this, what should my trigger look like?